>>43003289my main file is cubone + eevee (evolved to glaceon) and it's pretty curbstompy now with blizzard, bonemerang and earth power spam. bone rush on bosses when you can get up close. eevee was a decent partner throughout the game but nothing amazing. having refresh on
her was really useful, so I'd recommend that. they also get helping hand which is a party-wide +1 atk/sp.atk buff for the floor which is a strong option.
if you can't get steamroller early on, make sure to get a smackdown tm for cubone so you can hit fliers with your ground moves.
if I wanted to minmax with cubone I'd probably take pikachu to hit fliers with discharge, but you can also use jolteon or glaceon for that, as well as have the aforementioned support moves.